1. Field of the Invention
The present invention relates to a data recording medium, a data recording/reproducing apparatus and a data recording/reproducing method for recording/reproducing variable-bitrate-compressed video data and audio data synchronized with the video data in each unit of a fixed length. More particularly, the present invention relates to a data recording/reproducing apparatus and a data recording/reproducing method that rapidly performs a recording/reproducing operation of video data, in particular, a trickplay mode operation such as fast forward reproducing and rewind reproducing by using the data recording medium and also performs a postrecording operation to rewrite the audio data alone.
2. Description of the Related Art
Recently, image coding techniques for variable-bitrate-compression of video data in units of a picture have been studied in order to attain fast transfer of mass digital video data and recording of the mass digital video data in a data recording medium having a limited capacity. The MPEG standard, which is one of the image coding techniques, prescribes a compression method for digital video data by combining an I picture that is intra-picture (intra-frame or intra-field) compression coded and a P picture or a B picture that is inter-picture (inter-frame or inter-field) compression encoded. Such a data compression technique realizes the compression of digital video data from, for example, 150 Mbps into 3 Mbps.
In this method, however, the compression ratio of the digital video data depends upon the contents of the video data and the image correlations within the previous and subsequent pictures, and hence, each picture has a largely different capacity. For example, when the average data rate of the compressed video data is 3 Mbps, an image with little motion is compressed into 1 Mbps, while an image with much motion can be compressed into only 10 Mbps. Further, an I picture, a P picture and a B picture forming a series of scenes have respectively different capacities: on the average it is regarded that the I picture has a capacity approximately twice as large as that of the P picture and approximately four times as large as that of the B picture on the average. Therefore, when video data that is variable-bitrate-compressed in units of a picture is recorded on continuous areas on a medium such as an optical disk, where sectors each having a fixed length of, for example, 2 KB (2048 bytes) are previously formed, the location for each picture is largely varied. When such a data recording medium storing the compressed video data is used to perform a trickplay mode operation (such as the fast forward reproducing and the rewind reproducing) using the I picture data alone, it is indispensable to efficiently manage the location data on the I picture data which is located on the recording medium at unequal intervals.
In the MPEG standard, data is compressed in units of a GOP (group of pictures) including one I picture and a plurality of P pictures and B pictures. For example, when an NTSC signal for displaying approximately 30 pictures per 1 second is compressed into units of a GOP including twelve pictures, video data per 1 second includes 2.5 I picture data. Therefore, when a movie title of 135 minutes is compressed under this condition, the title includes as many as 20,250 I picture data. For recording all location data of such a large amount of I picture data using an address of four byte length in one continuous area of a medium, the size of this area is approximately 80 KB. Considering that the buffer capacity for an audio decoder forming a target decoder system described in the MPEG standard is limited within only 4 KB, the 80 KB capacity of a buffer for storing the location data of all I picture data makes it significantly difficult to realize a compact system.
Further, when variable-bitrate-compressed video data and audio data synchronized with the video data are interleaved in units of a sector to be recorded on a recording medium, the audio data is located on the recording medium at unequal intervals even if the audio data is a fixed length data obtained by non-compression coding or constant-bitrate-compression coding. Accordingly, when a rewritable data recording medium where such video data and the audio data are previously recorded is used to rewrite the audio data alone, i.e., to perform a postrecording operation, it is indispensable to efficiently manage the location data of the audio data which is located on the medium at unequal intervals.
For example, when stereo audio data is compressed in accordance with the MPEG standard into the data rate of 224 Kbps, i.e., 28 KB/s, a movie title of 135 minutes includes 226,800 KB of compressed audio data. When the compressed audio data is interleaved with the video data in units of a 2 KB sector to be recorded on a data recording medium, the compressed audio data is dispersedly recorded in 113,400 sectors. For recording all location data of such a large amount of audio data using an address of four byte length in one continuous area of a medium, the size of this area is approximately 450 KB. Further, when the entire audio data in a GOP is recorded in one continuous area, the number of the audio data is identical to that of the I picture data. Therefore, for recording all location data of these audio data in one continuous area, the size of this area is approximately 80 KB. The 450 KB or 80 KB capacity of a buffer is too large as compared with the limitation of buffer capacity for an audio decoder as described above, resulting in restricting the design of compact systems.